Photoelectric system and apparatus



Nov. 4, 1947.

J. SHONNARD PHOTO-ELECTRIC SYSTEM AND APPARATUS Filed Jan. 8, 1944 .PZMEEDU NOE INVENTOR.

J.R. SHONNARD Patented Nov. 4, 1947 PHOTOELECTRIC SYSTEM AND APPARATUS John R. Shonnard, New York, N. Y., assignor to Times Facsimile Corporation, a corporation of New York Application January 8, 1944, Serial No. 517,603

2 Claims.

This invention relates to photo-electric systems and apparatus for the conversion of light variations into electrical currents, useful in telefacsimile transmission and related arts.

A principal object of the invention is to provide an improved scanning arrangement for facsimile systems and for electro-optical systems generally.

Another object Off the invention is to provide systems or apparatus of the above-stated character in which substantially perfect fidelity in response is obtained; i. e., direct proportionality between the intensity of incident light and the current output from the converting or modulating system.

Stated in another Way, it is an object of the invention to minimize the harmonics or parasitic currents which are present in the output of photocell modulators and which are of sufficient magnitude to be troublesome at the lower output values. 1

In a preferred form of optical scanning apparatus for the transmitter of a tele-facsimile system, a balancing arrangement is employed by means of which an alternating-current carrier can be impressed upon the photo-cell or cells in such a manner that the output current from the scanning system is a constant-frequency carrier, the amplitude of which is a function of the excitation of the light-sensitive cell. It is desirable to obtain linearity between the variations in incident light and the signal output of the transmitter in order to avoid distortion of the modulated picture currents. The most effective modulator arrangement of this character in general use, described in the prior patent of Austin G. Cooley, N. 2,298,466 (dated October 13, 1942), employs a capacity-resistance bridge including the inherent capacity between the electrodes of the photo-cell or cells to translate light signals into current variations of an applied carrier current. However with this arrangement, if in assembling the apparatus the circuit constants are adjusted until an exact balance is obtained with no light illuminating the cell, the system is slightly unbalanced when the cell is illuminated and the light-input current-output relation is not exactly linear over the entire range of signal variation. Since it is an object of the present invention to obtain greater linearity in this relation, in this respect it constitutes an improvement over the arrangement shown in said prior patent.

Other objects and advantages of the invention will appear from the following description of the preferred embodiments thereof shown on the accompanying drawings, wherein Fig. 1 is a circuit diagram of a photo-electric system embodying features of the invention, and including elements of a tele-facsimile transmitter with which it is especially adapted to be used;

Fig. 2,is a graph illustrating the underlying principle of the invention; and

Fig. 3 is a diagram similar to that shown in Fig. 1 of a modified system employing a photocell having a single anode instead of two anodes.

Referring to Fig. 1, there is shown in schematic form the photo-electric components of a telefacsimile transmitter comprising a conventional rotating drum i0 carrying the picture or copy to be transmitted; an exciter lamp II and condensing lens [2 for illuminating an elemental area of the copy; and a photo-cell l3 for generating a signalling current modulated in accordance with the variations in shading of the successive elemental areas of the copy wrapped around the drum In. It will be understood that the invention is not limited to the photo-electric system of a tele-facsimile transmitter but may be used in electro-optical systems generally where a modulated carrier is generated.

The light-sensitive cell I3 is shown schematically since the structural details of such cells are well-known, and comprises a cathode 14 having an emissive coating on one side thereof and a plurality of anodes l5, l6 on opposite sides of said cathode. The principal or operative anode adjacent the coated surface of the cathode M consists of two spaced wires or strips of non-emissive metal l5 near the edges of the cathode so that they do not impede the passage of light to the cathode. The anode IE on the other hand, being on the rear of the cathode, is shielded from the active surface thereof. The geometric relation between the electrodes of the cell is such that the electrostatic capacity between the anode l5 and the cathode I4 is substantially equal to that between the anode l6 and the cathode M. A preferred cell of this type is the RCA #1645 photo-cell which is a highly-evacuated rubidium cell but any other suitable cell may be employed, depending upon the spectral response or other characteristic desired.

The output circuit of the cell I3 includes a conductor l9 connected to the control grid of an amplifier tube 20. The desired output current is a carrier of constant frequency, amplitudemodulated in accordance with the variations in the illumination of the cell l3.

3 In accordance with the invention, a source of alternating electromotive force 22 is connected to the cell and output circuit through a balancedbridge arrangement including as two arms thereof the inherent capacities between the respective anodes i5, I6 and the cathode M of the cell.

This arrangement is similar to that described in the above-mentioned patent of A. G. Cooley and, since the operating principles involved are fully set forth in said patent, only a brief description of the specific embodiment herein shown will be required for a complete understanding of the present invention. It will be apparent that the other two arms of the bridge are composed of the series resistors 23, 24, 25 and 26, together with a potentiometer 21 in shunt relation to the resistors 24 and 25. The grounded contact of the potentiometer permits balancing the bridge (zero output current) for either maximum or minimum light intensity on the cell, thus enabling the picture to be sent as either a positive or a negative, as desired.

The midpoint of the resistance section of the bridge is connected through conductor 28 and resistors 29 and 30 to the cathode [4 of the photocell I3. Any discrepancy between the interelectrode capacities of the cell may be corrected by the trimmer condenser 3|. As explained in the above-mentioned patent, the varying illumination of the cell during operation results in a carrier current modulated in accordance with the light signals in the cathode or output circuit. This modulated carrier current is amplified in the amplifier 20, further filtered and amplified if necessary in the filter 34 and amplifier 35, and then put on the line 36 or other transmission channel. The described system provides a signal which is adapted to operate conventional facsimile receiving apparatus to reproduce the picture or copy on the transmitting drum It]. An example of a suitable facsimile receiver is shown in the patent to A. G. Cooley, No. 1,702,595 granted Feb. 19, 1929. The amplitude of the transmitted signal is indicated by the power-level meter 31 bridged across the line output circuit 36.

In accordance with the invention, improved fidelity in response of the modulating arrangement described is obtained by connecting a steady bias potential of the order of 0.5 volt to the terminals of the cathode l4 and one of the anodes of the cell l3. As shown, this may be accomplished by connecting a positive bias of a fraction of a volt derived from the bleeder circuit 38 through a resistor 39 to the anodes l5, 15. Since the cathode I4 is substantially at ground potential because the voltage drop in the resistors 21, 29 and 30 is negligible, the anodes l5, l are biased with respect to the cathode M in accordance with the drop across the portion of the bleeder 38 between the tap and the grounded end.

of the bleeder. It is to be noted that while in the drawing, the bias potential is shown as applied to the anodes l5, l5; it is preferred to utilize a carrier source 22 having a low internal directcurrent resistance so that the anodes l5 and 16 are conductively connected together and it is immaterial whether the bias potential derived from the bleeder circuit is connected to the anodes I5, l5 or to the anode l6. Obviously the steady bias potential may be derived from any available source of the proper value.

The improved result obtained by the invention may be at least partially explained by the relation between the photocell current and applied voltage at the terminals of the cell, as shown by the graph of Fig. 2 for constant illumination of the cell. While the illumination of the cell varies over a wide range in actual operation, the condition illustrated will be encountered at times, and when the light excitation is greater or less, the effect will be similar although it may be less pronounced under certain conditions. In the graph, the relation between the photocell current and the applied voltage at any instant is represented by the curve 4i and the carrier potential applied to the cell is represented by the curve 42. As indicated, the amplitude of the carrier potential is preferably less than that which would cause rectification in the cell. It will be seen that the photocell current varies in direct proportion to the applied voltage from slightly less than zero to approximately plus 0.75 volt in the case of the #1645 rubidium cell. If no bias potential is employed, the negative peaks of the carrier potential will extend beyond the straight-line portion of the voltage-current curve, and harmonics will be generated in the output circuit of the cell. These harmonics or parasitic modulation currents although of small magnitude are appreciable, especially at the lower values of light excitation.

When however a bias potential of between 0.3 and 0.5 volt and of the proper polarity is impressed on the cell, displacing the axis of the carrier wave 42 to the center of the straight-line portion of the current-voltage curve 4| as indicated in Fig. 2, the peaks of the carrier wave lie substantiallywithin the straight-line portion of the curve 4! and the output modulation current contains no harmonics. In this manner direct proportionality between light excitation and output current is maintained by the modulator arrangement.

The optimum value of the bias potential depends upon the construction of the photocell and upon the light-sensitive coating employed on the cathode. Commonly the photocell cathodes comprise or are coated with one of the alkali metals or alloys thereof, and so far as I have been able to ascertain the optimum bias potential for any of the cells commercially available at the present time is a fraction of a volt and lies between 0.2 and 0.6 volt. The precise value of bias to be employed with any particular cell may be readily determined by trial, for example with as oscilloscope connected in the output circuit of the modulator.

The invention is not limited to modulating systems employing a balancing anode, similar to the RCA #1645 referred to above, but may also be utilized in modulator systems employing the usual cell having a single anode. Such a system is illustrated in Fig. 3, wherein a photocell 45 having a cathode 46 and an anode 41 is shown, said cell having no balancing anode corresponding to the anode [B in Fig. 1. The output circuit 48 of the modulator is shown as connected to the cathode 46 A source 5| of alternating electromotive force is connected to the cell through a balanced bridge arrangement in order to produce a carrier current in the output circuit 48, the magnitude of which depends upon the lightexcitation of the cell 45. This bridge arrangement comprises a resistance network 52 generally similar to that shown in Fig. l, the midpoint of which is connected through a conductor 53, and resistors 54 and 55 to the cathode 45 of the cell. The capacity arms of the bridge consist of the inherent capacity between the electrodes of the cell and the capacity of the condenser 56 which is adjusted to match the interelectrode capacity of the cell. In

accordance with this modification of the invention, a positive bias potential is applied to the anode 41 of the cell through a resistor 58 to obtain linearity between light excitation and current output, for example from a battery 59 producing a steady potential of the order of 0.5 volt. Any other source of steady potential of the proper value may of course be substituted for the battery 59, and since the negative terminal of the battery is grounded the potential thereof biasses the anode ll approximately 0.5 volt with respect to the cathode 46. The precise value of bias potential to be employed for optimum results depends, as explained above, upon the construction and cathode coating of the cell 45.

The invention is not limited to the modifications shown and described above for the purpose of explaining the invention, since various changes will occur to those skilled in the art which do not involve a departure from the scope of the invention as defined in the appended claims.

I claim:

1. In a scanning system for a tele-facsimile transmitter, in combination, means including a photocell for scanning the copy, said cell being provided with a cathode, an operative anode in front of the cathode and a balancing anode shielded from the electron emission of the cathode, an output circuit for said cell, a source of alternating electromotive force, a bridge network connected between said source and the cell so that an alternating-current carrier is produced in the output circuit of the cell which varies in accordance with the illumination of the cell and means for applying a steady positive bias potential of the order of 0.5 volt to the operative anode with respect to the cathode, said bias potential being insufiicient to cause rectification in the cell,

2. In a scanning system for a tele-facsimile transmitter, in combination, means including a photocell for scanning the copy, said cell being provided with a cathode, an operative anode and a balancing anode, an output circuitfor said cell, a source of alternating electromotive force, a bridge network connected between said source and the cell so that an alternating-current carrier is produced in the output circuit of the cell which varies in accordance with the illumination JOHN R. SHONNARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,219,676 Barber Oct. 29, 1940 1,871,994 Iams Aug 16, 1932 2,162,792 Teves June 20, 1939 2,155,034 Barthelemy Apr. 18, 1939 2,298,466 Cooley Oct. 13, 1942 1,546,504 Moorhead July 21, 1925 1,688,292 Weaver Oct. 16, 1928 

